Hydroxy-containing ethylenically unsaturated polyesters prepared from glycidyl polyethers of polyhydric phenols and ethylenically unsaturated monocarboxylic acids tend toward premature gellation. In other words, the polyesters, if stored for long periods of time before use, will increase in viscosity (gel) to a value which severely limits their use for many applications where low viscosity is important.
Such premature gelation is significantly reduced by the addition of a dialkylhydroxylamine. See, for example, U.S. Pat. No. 3,408,422, issued Oct. 29, 1968.
When, however, the epoxy compound contains tertiary, allylic or benzylic hydrogens, such as the so-called saturated epoxy resins and epoxidized novolac resins, there appears to be a propensity toward auto-oxidation and peroxide formation. Accordingly, when such epoxy compounds are esterified with ethylenically unsaturated acids, the reaction mixture gels during the esterification step. It has now been found that if the saturated epoxy resin or epoxidized novolac resin is pretreated with a trialkylphosphite before the esterification, the esterification can be effected without premature gelation to produce unsaturated vinyl esters exhibiting excellent stability.